Backup battery charging circuit

ABSTRACT

An object of the invention is to provide a backup battery charging circuit capable of continuing to charge a backup battery to the limit of the voltage drop of a main battery so as to reduce the possibility of data disappearance to an achievable extent. For this purpose, a backup battery charging circuit comprises: a main battery serving as a power supply; a backward current protection diode; a regulator circuit using the main battery as the power supply; a limiting resistor; a backup battery; a switch for performing direct charging from the main battery; and a comparator for determining whether the regulator circuit or the switch is to be used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backup battery charging circuit forcharging a backup battery used for the backup of data in a memory, amicrocomputer, or clock function.

2. Related Art of the Invention

With recent spread of portable equipment, various apparatuses comprisingclock function and the like require a backup battery for maintaining thenecessary function even when a main battery serving as a power supply isremoved and even when the voltage of the main battery goes lower.

When the main battery is removed and when the voltage of the mainbattery goes lower, the function of a charging current supply for thebackup battery is lost. Thus, the voltage of the backup battery goeslower owing to the current consumption by a device that uses the backupbattery as the power supply. When the voltage of the backup battery goesdown to a certain voltage or lower, data disappears in the memory, themicrocomputer, or the clock function that use the backup battery as thepower supply. In order to reduce the possibility of such datadisappearance, the backup battery is charged as much as possible duringnormal operation.

A backup battery charging circuit according to the prior art hasconfiguration as shown in FIG. 2. In FIG. 2, numeral 1 indicates a mainbattery (such as a secondary battery) serving as a power supply. Numeral2 indicates a backward current protection diode. Numeral 3 indicates aregulator circuit. Numeral 4 indicates a limiting resistor. Numeral 5indicates a backup battery.

The operation of this backup battery charging circuit is describedbelow. The regulator circuit 3 receiving the power in the form of theelectric potential obtained as the voltage V_(MB) of the main battery 1minus the forward voltage V_(T) of the backward current protection diode2 supplies a charging current to the backup battery 5. The backwardcurrent protection diode 2 prevents that a current leaks from the backupbattery 5 to the main battery 1 when the voltage of the main battery 1goes lower. The limiting resistor 4 avoids an excessive chargingcurrent.

Since the withstand voltage of the backup battery 5 is lower than thevoltage V_(MB) of the main battery 1, the output voltage V_(RG) of theregulator circuit 3 is set at a voltage not exceeding the withstandvoltage of the backup battery 5.

Nevertheless, when the voltage V_(MB) of the main battery 1 goes lower,the output voltage V_(RG) of the regulator circuit 3 goes lower as shownin FIG. 4. FIG. 4 shows the electric potential obtained as the mainbattery voltage V_(MB) minus the forward voltage V_(T) of the backwardcurrent protection diode 2, the voltage V_(BU) of the backup battery 5,and the output voltage V_(RG) of the regulator circuit 3. The interval(1) indicates a backup battery charging interval.

At the time when the output voltage V_(RG) of the regulator circuit 3goes lower than the voltage V_(BU) of the backup battery 5, the chargingbecomes impossible. After that, the electric charge in the backupbattery 5 is continuously consumed by the memory, the microcomputer, orthe clock function. This causes a possibility of data disappearanceunless the backup battery 5 is recharged.

SUMMARY OF THE INVENTION

The invention has been devised in order to resolve above-mentionedproblem in the prior art. An object of the invention is to provide abackup battery charging circuit capable of continuing to charge a backupbattery to the limit of the voltage drop of a main battery so as toreduce the possibility of data disappearance to an achievable extent.

In order to achieve this object, a backup battery charging circuitaccording to the invention comprises: a main battery; a regulatorcircuit for receiving electric power from the main battery so as tosupply a charging current to a backup battery; a direct charging circuitfor supplying a charging current from the main battery to the backupbattery without the intervention of the regulator circuit; and a modedetermining means for determining an appropriate charging mode from thegroup consisting of a first charging mode where the charging current issupplied from the regulator circuit to the backup battery and a secondcharging mode where the charging current is supplied from the mainbattery to the backup battery without the intervention of the regulatorcircuit, so as to charge the backup battery.

This mode determining means selects the first charging mode, forexample, when the voltage of the backup battery is lower than or equalto the output voltage of the regulator circuit, and selects the secondcharging mode when the voltage of the backup battery exceeds the outputvoltage of the regulator circuit.

As described above, the backup battery charging circuit according to theinvention comprises the second charging mode in which the charging isperformed directly from the main battery without the intervention of theregulator circuit and which is used when the voltage of the main batterygoes lower, in addition to the first charging mode in which the chargingis performed with the output voltage of the regulator circuit and whichis used when the voltage of the main battery is high. Then, anappropriate charging mode is selected from the first charging mode andthe second charging mode, so that the backup battery is charged. Thispermits the continuation of the charging of the backup battery to thelimit of the voltage drop of the main battery, and hence permitsefficient charging so as to reduce the possibility of data disappearanceto an achievable extent.

More specifically, this backup battery charging circuit comprises: amain battery serving as a power supply; a backward current protectiondiode; a regulator circuit using the main battery as the power supply; alimiting resistor; a backup battery; a switch (direct charging circuit)for performing direct charging from the main battery; and a comparator(mode determining means) for determining whether the regulator circuitor the switch is to be used.

The comparator compares the output voltage of the regulator circuit withthe voltage of the backup battery, so as to perform charging from theoutput voltage of the regulator circuit when the voltage of the backupbattery is lower than or equal to the output voltage of the regulatorcircuit, and perform direct charging from the main battery when thevoltage of the backup battery exceeds the output voltage of theregulator circuit.

This configuration comprises the charging mode in which the charging isperformed with the voltage of the main battery through the backwardcurrent protection diode to an achievable extent and which is used whenthe main battery voltage goes lower, in addition to the other chargingmode in which the charging is performed with the output voltage of theregulator circuit and which is used when the voltage of the main batteryis high. This permits efficient charging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a backup batterycharging circuit according to an embodiment of the invention.

FIG. 2 is a block diagram showing the configuration of a backup batterycharging circuit according to the prior art.

FIG. 3 is a characteristic diagram showing a backup battery chargingcharacteristic according to an embodiment of the invention.

FIG. 4 is a characteristic diagram showing a backup battery chargingcharacteristic according to the prior art of a backup battery chargingcircuit.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention is described below in detail withreference to the drawings.

FIG. 1 is a block diagram showing the configuration of a backup batterycharging circuit according to an embodiment of the invention. Here,components that correspond to those described in the prior art of FIG. 2and that have essentially the same function as the described one aredesignated by the same numerals. In FIG. 1, numeral 1 indicates a mainbattery (such as a secondary battery). Numeral 2 indicates a backwardcurrent protection diode. Numeral 3 indicates a regulator circuit.Numeral 4 indicates a limiting resistor. Numeral 5 indicates a backupbattery. Numeral 6 indicates a charging switch. Numeral 7 indicates acomparator.

The operation of the backup battery charging circuit according to thisembodiment is described below with reference to FIG. 1.

When the voltage obtained as the voltage V_(MB) of the main battery 1minus the forward voltage V_(T) of the backward current protection diode2 is within a range permitting the operation of the regulator circuit 3having a setting output voltage (for example, 3.0 V) that is determinedwith taking into account the withstand voltage of the backup battery 5,the backup battery 5 is charged to 3.0 V at maximum. At this time, thecharging switch 6 is off.

When the voltage V_(MB) of the main battery 1 goes lower, the supplyvoltage applied through the backward current protection diode 2 onto theregulator circuit 3 also goes lower. As a result, at the time pointwhere the dynamic range has been narrowed to an extent that the settingvoltage cannot be outputted, the output voltage V_(RG) of the regulatorcircuit 3 begins to go lower. When the output voltage V_(RG) of theregulator circuit 3 is greater than or equal to the voltage V_(BU) ofthe backup battery 5, the backup battery charging from the regulatorcircuit 3 is performed. Nevertheless, at the time when the outputvoltage V_(RG) of the regulator circuit 3 goes lower than the voltageV_(BU) of the backup battery 5, the backup battery charging from theregulator circuit 3 becomes impossible.

At this time, the comparator 7 compares the output voltage V_(RG) of theregulator circuit 3 with the voltage V_(BU) of the backup battery 5, soas to turn on the charging switch 6 when the output voltage V_(RG) ofthe regulator circuit 3 goes lower than the voltage V_(BU) of the backupbattery 5. This permits the continuation of the charging with thevoltage obtained as the voltage V_(MB) of the main battery 1 minus theforward voltage V_(T) of the backward current protection diode 2. Thispermits the continuation of the charging of the backup battery 5 to thelimit of the voltage drop of the main battery 1, as shown in FIG. 3.FIG. 3 shows the electric potential obtained as the main battery voltageV_(MB) minus the forward voltage V_(T) of the backward currentprotection diode 2, the voltage V_(BU) of the backup battery 5, and theoutput voltage V_(RG) of the regulator circuit 3. The interval (1)indicates an interval of backup battery charging from the regulatorcircuit 3, while the interval (2) indicates an interval of backupbattery charging through the charging switch 6.

For example, when the present voltage of the main battery 1 is 2.0 V,and when the forward voltage V_(T) of the backward current protectiondiode 2 is 0.3 V, the backup battery 5 can be charged to a voltage of1.7 V (2.0 V-0.3 V).

It should be noted that the charging mode using the charging switch 6 isperformed in the interval where the voltage of the main battery 1 islow. Thus, this voltage does not exceed the withstand voltage of thebackup battery 5.

1-2. (canceled)
 3. A backup battery charging circuit comprising: aregulator circuit for receiving electric power from a main battery so asto supply a charging current to a backup battery; a diode for beingconnected directly to the main battery, for preventing leakage of acurrent from said backup battery to said main battery; a direct chargingcircuit for supplying a charging current from said main battery to saidbackup battery without the intervention of said regulator circuit; and acharging mode determiner for determining an appropriate charging modefrom the group consisting of a first charging mode, where the chargingcurrent is supplied through said regulator circuit to said backupbattery, and a second charging mode, where the charging current issupplied from said main battery to said backup battery without theintervention of said regulator circuit, so as to charge said backupbattery, wherein: said charging mode determiner compares the outputvoltage of said regulator circuit with the voltage of said backupbattery, selects said first charging mode when the voltage of saidbackup battery is lower than or equal to the output voltage of saidregulator circuit and selects said second charging mode when the voltageof said backup battery exceeds the output voltage of said regulatorcircuit.
 4. A backup battery charging circuit according to claim 3,wherein said direct charging circuit includes a switch for being placedbetween said main battery and said backup battery, in order to connectsaid main battery directly to said backup battery during said secondcharging mode.
 5. A backup battery charging circuit according to claim3, wherein said charging mode determiner includes a comparator thatinputs the output voltage of said regulator circuit and the voltage ofsaid backup battery, the output signals of said comparator controllingthe conducting and the blocking of current to said backup battery bysaid switch.
 6. A backup battery charging circuit according to claim 5,wherein said comparator is connected to said switch, for turning on saidswitch during said second charging mode.